1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to systems and techniques for receiving signals over a wide range while preventing interference from intermediate frequency (IF) images with desired signals.
2. Description of the Related Art
Reception of electromagnetic signals is effected by a transmission and reception of signals in a first relatively high frequency band (such as radio frequency or xe2x80x98RFxe2x80x99) and subsequent downconversion of the received signal to a lower frequency band (such as IF.) Downconversion is typically achieved by mixing the received signal with a reference signal generated by a local oscillator (LO). For example, an RF signal received at a frequency of 1000 kilohertz (kHz) might be mixed down to 100 kHz by a mixer having a 900 kHz reference signal from a local oscillator. Under these conditions, a signal at 800 kHz would have the same IF (intermediate frequency) and would therefore interfere with the desired signal at 1000 kHz. When mixing two signals, provision must be made to prevent the mixing image from interfering with the desired signal. At least three techniques are known in the art for minimizing the interfering effects of mixing images.
A first technique involves the use of a narrowband filter in front of the mixer. Unfortunately, this approach suffers from the requirement of a customized design for each desired frequency of operation.
A second approach involves the use of tuned amplifiers, instead of filters, in front of the mixer. Unfortunately, it has been difficult to achieve consistent gain and bandwidth rejection over a wide range in a tuned amplifier. In addition, slight variations in component characteristics between amplifiers disposed in separate paths in the mixer impede the ability of the amplifiers to track each other with sufficient accuracy for many demanding applications.
In accordance with a third approach, image rejecting mixers using passive phase shifters are used in place of filters. Passive phase shifting networks shift phase over a single frequency or a narrow band of frequencies. Subsequently, changing the output frequency requires a redesign or retuning of the network, both of which add cost and complexity to the system. This is particularly problematic in a frequency-hopping environment where external tuning networks could be used but would slow the speed at which frequency hopping would be permitted.
Hence, a need exists in the art for an improved, yet inexpensive system or technique for rejecting mixing images in radio frequency receivers over a wide frequency range.
The need in the art is addressed by the system and method of the present invention for effecting wideband image rejection. In a receiver implementation, the inventive method includes the steps of receiving a first signal in a first frequency band and generating in-phase and quadrature signals therefrom. The phase of the in-phase signal is shifted to provide a second signal and the phase of the quadrature signal is shifted to provide a third signal. A predetermined phase relationship is thereby effected between the second and the third signals. The second and third signals are then summed to provide an output signal which has minimal interference from a mixing signal.
In the illustrative embodiment, the phase shifting is achieved via the use of all pass networks. In an illustrative implementation, each of the all pass networks include a differential amplifier having first and second input terminals. The first and the second terminals are connected to a first end of first and second resistors, respectively. The second ends of the first and second resistors are connected to a common input terminal for the network. The first input terminal is a negative terminal and is connected to an output terminal of the network. The second terminal is a positive terminal and is connected to a source of ground potential via a capacitive element.
Whether implemented in a receiver or as a general-purpose mixer, the inventive mixer offers a wide-band output stage and affords good gain matching over a wide bandwidth. This allows good image rejection over a wide bandwidth. Hence, the inventive mixer may be used in high-speed frequency hopping applications without substantial changes in the design thereof.